It is well established that the primary modes of altering myocardial contractility on a beat-to-beat basis involve both the modulation of the myoplasmic Ca2+ ([Ca2+]i) availability at the level of the myofilaments (MF), and changes in the Ca2+-response of the MF, especially via length- dependent activation. Alternatively, certain more slowly developing covalent modifications of MF have been recognized (such as thick or thin filament phosphorylation), but these are of uncertain functional significance in cardiac tissue. Hypothesizing that the [Ca2+]i "history" regulates contractility via such covalent modification, we performed [Ca2+]i-clamp experiments at various levels (ranging up to 10-fold > resting [Ca2+]i) and durations (up to 60s) via 10 hz tetanization in sarcoplasmic reticulum-disabled (thapsigargin or caffeine treated) intact rat cardiac myocytes loaded with indo-1 free acid. We observed that the rising phase of the tetanus (2-3 sec) was sufficiently slow such that Ca2+-MF binding, and thus force and cell length, were throughout at equilibrium, enabling a rapid assessment of baseline contractile activation. A secondary [Ca2+]i- and time-dependent increase in cell shortening was observed despite steady levels of clamped [Ca2+]i, and without significant change in pHi, Mgi or ATPi (as assessed by SNARF and mag-indo-1 loaded cells). This secondary leftward shift in the length-pCa curve was attenuated by the calmodulin inhibitors chlorpromazine, W7, and Sr2+, and by the myosin light chain kinase (MLCK) inhibitor KT5926, and was amplified by the phosphoprotein phosphatase inhibitor Calyculin A. We propose that [Ca2+]i history, via Ca2+-calmodulin-dependent MLCK activity, myosin light chain phosphorylation, and kinase/phosphatase balance, plays a significant modulatory role in the chronic regulation of contractility in intact heart cells. This integrative phenomenon would reflect a history dependence of cardiac work/demand, and may serve as an important cardiovascular adaptive mechanism in myocardial conditioning.